In the Democratic primary, with five candidates, Murphy enjoys a strong advantage:

Patrick Murphy: 55 percent;

Alan Grayson: 22 percent;

Pam Keith: 4 percent;

Roque De La Fuente: 0 percent;

Reginald Luster: 0 percent;

Undecided: 19 percent.

Rubio, already elected statewide, is widely known and well-liked:

Recognize/Favorable: 44 percent;

Recognize/Unfavorable: 35 percent;

Recognize/Neutral: 19 percent;

Don't Recognize: 2 percent.

While Murphy is less known, but his unfavorable level is lower:

Recognize/Favorable: 30 percent;

Recognize/Unfavorable: 11 percent;

Recognize/Neutral: 38 percent;

Don't Recognize: 21 percent.

Although Republican presidential candidate Donald Trump is expected by some to have a negative impact on GOP Senate candidates, Coker expects Rubio to have one advantage:

"Rubio can keep the Hispanic voters that Trump is losing," Coker said.

At a GOP fundraising event in Palm Beach County, Rubio gave a speech portraying his Senate race as a vital protection against a Democratic majority in the upper chamber of Congress.

"If we lose the Florida Senate race, we put the balance of power in danger in the U.S. Senate," he said, according to The Washington Post.

"And even if we win the presidency, you won't be able to get anything done because Chuck Schumer is blocking it. We cannot lose the Senate. And we cannot lose Florida," he said, referring to the senior senator from New York.

Mason-Dizon Polling surveyed 625 registered voters in Florida by phone, from Aug. 22-24, with a margin of error of +/- 4 percentage points.